Coordinated inhibition of RNA Polymerase I and the PI3K/AKT/mTOR signalling pathway targets ribosome biogenesis and ribosome function to provide a novel and potent therapeutic approach to treat MYC driven cancer

© 2014 Dr. Jennifer Rose DevlinThe growth and proliferation of both normal and malignant cells is dependent on the sustained synthesis of new proteins by ribosomes. Ribosome biogenesis is the tightly regulated multi-step process through which new functional ribosomes are assembled from ribosomal RNAs (rRNAs) and ribosomal proteins (RPs). Importantly the synthesis of ribosomes and ribosomal components is highly coordinated by transcriptional networks and signalling pathways that are drive proliferative growth. These networks and pathways are frequently dysregulated in human cancer, underpinning the near ubiquitous dysregulation of ribosome biogenesis observed in transformed cells. The hyperactivation of ribosome biogenesis and protein translation in cancer cells, in particular those driven by dysregulated c-MYC activity, suggests that they will be vulnerable to therapeutic strategies that target these essential growth processes. This thesis investigates the hypotheses that i) the PI3K/AKT/mTOR signalling pathway plays a critical role in the promotion of ribosome biogenesis and ribosome function in MYC-driven cancer cells and; ii) that the coordinated targeting of ribosome biogenesis and ribosome function will cooperate to improve therapeutic outcomes in B-lymphoma driven by c-MYC over-expression. Signalling mediated by the protein kinase AKT was demonstrated to be critical for ribosome biogenesis via control of both rRNA and RP synthesis, in MYCdriven B-lymphoma cells. Disruption of these essential cellular growth processes following the targeted inhibition of AKT signalling resulted in the delay of disease progression in mice with transplanted MYC-driven Blymphoma. Furthermore, disruption of Pol I transcription mediated by the targeted blockade of the PI3K/AKT/mTOR signalling pathway was linked to the induction of B-lymphoma cell apoptosis. Importantly the potency of different inhibitors in suppressing the PI3K/AKT/mTOR signalling pathway correlated with their potency in reducing the rate of Pol I transcription and cell viability. Selective inhibition of Pol I transcription mediated B-lymphoma cell death via the nucleolar stress response pathway driving the accumulation of free RPs, increased RP-MDM2 binding and robust activation of the p53 pathway. Surprising B-lymphoma cell death following PI3K/AKT/mTOR pathway inhibition was not mediated via this canonical response pathway. PI3K/AKT/mTOR-inhibitor mediated apoptosis was instead achieved via the selective up-regulation of the intrinsic apoptotic machinery, specifically the accumulation of BMF, a pro-apoptotic BH3-only protein encoded by an IRES containing mRNA, the translation of which is promoted following inhibition of PI3K/AKT/mTOR-regulated CAP-dependent protein translation. Consistent with their ability to induce B-lymphoma cell death via distinct pro-apoptotic mechanisms, targeted inhibitors of the PI3K/AKT/mTOR signalling and Pol I transcription combined to enhance B-lymphoma cell apoptosis and extend the survival of mice with transplanted MYC-driven B-lymphoma. Thus, the coordinated inhibition of ribosome biogenesis (rRNA synthesis) and ribosome function (mTORC1-driven protein translation) cooperated to kill MYC-driven lymphoma cells as well as to significantly delay disease relapse and extend survival in a mouse model of MYC-driven malignancy. This work thereby identifies targeting of the ribosome at multiple points as a novel potential therapeutic approach for cancer treatment